Great question! The good news is that most major databases let you modify just the scale (the second number in DECIMAL(A,B)) without having to redefine all other column properties like default values or nullability. The exact syntax depends on your database system, so let's cover the common ones:

MySQL/MariaDB #

You can use the ALTER TABLE ... ALTER COLUMN statement directly, specifying only the new DECIMAL(A,C) definition. The database will automatically retain existing settings like NULL/NOT NULL and default values—you don't need to repeat them unless you want to change them too.

Example:

ALTER TABLE your_table ALTER COLUMN price DECIMAL(10, 4);

If price was originally DECIMAL(10,2) with a default value of 0.00 and allowed nulls, those properties stay intact after running this command.

Note: If you're reducing the scale (e.g., from 4 to 2), MySQL will warn you about potential data truncation by default. Make sure your data fits the new scale first, or adjust your sql_mode if you intentionally want to truncate values (not recommended without a data backup).

SQL Server #

Similar to MySQL, ALTER TABLE ... ALTER COLUMN works here. As long as you only specify the new DECIMAL(A,C) type, existing column attributes (nullability, defaults) are preserved.

Example:

ALTER TABLE your_table ALTER COLUMN tax_rate DECIMAL(18, 5);

If tax_rate was NOT NULL with a default of 0.000, those settings remain unchanged.

Note: SQL Server will throw an error if reducing the scale would truncate existing data. You’ll need to verify your data fits the new scale first, or use SET ARITHABORT OFF temporarily (use cautiously, as it can lead to silent data loss).

PostgreSQL #

PostgreSQL uses ALTER TABLE ... ALTER COLUMN ... TYPE to modify column types. Again, you only need to specify the new DECIMAL(A,C) definition, and existing properties like nullability and defaults are kept.

Example (increasing scale):

ALTER TABLE your_table ALTER COLUMN discount TYPE DECIMAL(8, 3);

If you need to reduce the scale and want to handle data safely (e.g., rounding instead of truncating), use the USING clause:

ALTER TABLE your_table ALTER COLUMN discount TYPE DECIMAL(8, 1) USING ROUND(discount, 1);

This ensures values are rounded to the new scale instead of causing an error.

Key Takeaway #

Across all these systems, the core idea is: you don't need to re-declare all column properties when only changing the scale of a DECIMAL column. Just specify the new DECIMAL(A,C) in your ALTER statement, and the database will retain the existing settings for nullability, default values, etc.

内容的提问来源于stack exchange，提问作者Kaan